Changes in the abundance, species richness and species diversity of zooplankton were studied in the Paraguay River. Fifty-nine sites were studied at two hydrological phases between Porto Cáceres (16º03'S-26º23'W) and the confluence with the Paraná (Argentine, 26º53'S-58º23'W), representing a distance of 2270 km. Zooplankton densities varied between 1 and 60 ind.l -1 at high water and between 11 and 100 ind.l -1 at low water. Multiple regression analysis revealed that the hydrological phase explained 64% of the variability in zooplankton density. Surveys found 196 taxa in the Paraguay River (including both the main course and its floodplain). The greater species richness was registered in the upper section. There were significant differences in the species richness and species diversity of Rotifera (Monogononta) between hydrological phases. In the main channel, the hydrological phase explained 54% of variability in species richness, whereas water temperature and electric conductivity explained less than 22% of the variability in species diversity. Rotifera was the most abundant group in both study periods. Despite the total number of species registered in the main channel, only six planktonic genera dominated the zooplankton assemblage (Polyarthra, Synchaeta, Filinia, Keratella and Lecane). The most abundant cladocerans belonged to Bosminidae (Bosminopsis sp.) and the dominant copepods were in the nauplii and juveniles stages. A longitudinal pattern in dominant taxa was not defined at high water. At low water, three species-site groups were separated in relation to environmental variables. The pulsing of the river determines the degree of connectivity with the floodplain and local features had a greater effect on zooplankton assemblages than large-scale landscape patterns.

The potential colonization by anchored plants (PCAP) and the potential areas for initial colonization of free floating plants were estimated during the early filling phase for the Yaciretá reservoir. In order to obtain the PCAP, the observed maximum depth of colonization of the anchored macrophytes before impoundment and the hypsographic curves were used. The species inhabiting the preimpoundment area were classified according to the different bioforms before the inclussion in the analysis. The areal extent of PCAP (from depths between 0-4 m) could reach 275 km 2 at 76 m above sea level (current water level), whereas at 82 m above sea level (final filling level) the littoral zone will be increased by about 21.5%. The potential area for geophytes was estimated to be 99 km 2 ; 131 km 2 for root-floating leaved plants and 120 km 2 for submerged plants, at 76 m above sea level. At 82 m above sea level, the geophytes could reach 271 km 2 . The data for wind frequency, velocity and fetch, together with depth were used to calculate shallow and sheltered areas in which free floating plants could find favourable conditions to initial colonization. Physical and chemical features recorded at eight stations during the early filling phase are discussed in relation to potential plant development.Key words: tropical rivers, impounding reservoirs, South America, potential macrophytes colonization, reservoirs. RESUMO Predição da colonização por macrófitas no reservatório de Yaciretá do Rio ParanáA colonização potencial por plantas enraizadas (CPPE) e as áreas potenciais de colonização por plantas fluentes livres foram calculadas durante a fase de enchimento do reservatório de Yaciretá. Para obter o CPPE, a máxima profundidade de enraizamento observada previa o fechamento do rio e as curvas ipsográficas foram utilizadas. As espécies presentas na área pré-represamento foram classificadas de acordo com as diferentes bioformas antes da inclusão na análise: A extenção de CPPE (para profundidades entre 0-4 m) poderia alcançar 275 km 2 a 76 m acima do nível do mar (nível de água atual), enquanto na cota de 82 m acima do nível do mar (nível de recheio final) a zona litorânea será aumentada em aproximadamente 21,5%. A área potencial para macróficas emersas foi calculada em 99 km 2 , para as plantas enraizadas com folhas flutuantes em 131 km 2 , e para as plantas enraizadas submersas em Rev. Brasil. Biol., 60(4)

We aim to analyze the spatial patterns of the vegetation at local and landscape scale during a scenario characterized by abnormally dry period (2007)(2008) and a reference scenario (1976)(1977). We used the frequency occurrence of the species and the number of species belonging to five life forms to compare the two temporal scenarios. In five water bodies of Esteros del Iberá we surveyed vascular aquatic macrophytes and the wetland plants during spring-summer with up to three samples of 20 m 2 each. A total of 161 species were found during the two study periods (120 and 117 species respectively). Cyperaceae and Poaceae were the most species-rich families at both periods. At waterbody scale, beta diversity showed a high spatial heterogeneity and at wetland scale, the turnover rate of S is low. At both scenarios, the vegetation composition showed relevant differences between the sub-rounded lakes and the elongated waterbodies. The floristic composition and the life forms spectra were affected by the change in the water supply. The knowledge of the baseline of the spatial and temporal variability of this particular wetland is key to support future assessments of the ecosystem response to the global change.